L22. Molar Mass

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L4. Molar Mass

How much does one mole of a substance weigh in grams?

Central Idea:

Molar mass is the mass of one mole of a substance, measured in grams per mole (g/mol). Calculate it by adding the atomic masses from the periodic table, using subscripts as multipliers (for example H2O uses two hydrogens and one oxygen).

How To Calculate:

1. Identify the formula and count each atom (read subscripts; H2O → H=2, O=1). 2. Look up atomic masses on the periodic table (use decimals: H ≈ 1.008, O ≈ 16.00). 3. Multiply each atomic mass by its subscript (if none, multiply by 1). 4. Add the results to get molar mass and include units g/mol. 5. Report appropriate significant figures (often round H2O to 18.02 g/mol).

Apply To New Situations:

Compare substances: does the name and the atoms remain the same before and after change? Try reversing a step in your reasoning (imagine separating products back to reactants). Check for gas, precipitate, or mass change. Measure mass and units to confirm molar mass calculations. Quick-check practice question: What is the molar mass of CO2 (use C = 12.01, O = 16.00)?

Synthesize Ideas:

Molar mass equals the sum of atomic masses per formula unit; use terms reactant, product, mole, and g/mol when describing calculations. Example calculation: H2O molar mass = 2 × 1.008 + 16.00 = 18.016 g/mol (report 18.02 g/mol). Another quick example: CO2 = 12.01 + 2 × 16.00 = 44.01 g/mol.

Checklist: check units (g, mol, g/mol), verify atom counts and subscripts, and confirm you added atomic masses correctly. Tell the atom-level story: which atoms are present and conserved in the formula. Reuse these steps on new problems to build confidence.

Key Takeaways:

Molar mass equals the mass of one mole of a substance expressed in grams per mole (g/mol); it is found by summing atomic masses from the periodic table according to the chemical formula, treating subscripts as multiplication factors for each element.
To calculate, count atoms from the formula, multiply each atomic mass by its subscript, then add the results and include units g/mol; round to sensible significant figures for reporting, as demonstrated when H2O is given as 18.02 g/mol.
Worked example: H2O has two hydrogens and one oxygen so molar mass = 2 × 1.008 + 16.00 = 18.016 g/mol, commonly rounded to 18.02 g/mol; always show units and the arithmetic steps in your work.
Apply this to CO2 by counting one carbon and two oxygens: molar mass = 12.01 + 2 × 16.00 = 44.01 g/mol; this quick application shows how subscripts change the multiplication step and therefore the final molar mass.
Verification checklist: confirm units, recount atoms and subscripts, add atomic masses correctly, and check that the calculation matches observable evidence or expected values; practice these steps to gain accuracy and confidence, and compare with a periodic table or reliable reference to check results across several problems until you consistently reach the same rounded molar masses and units each time reliably.